Process for treating metallic surfaces with an ionic beam



K. W. EHLERS ss FOR TREATING METALLIC SURFACES WITH AN IONIC BEAM Sept. 12, 1967 PROCE Filed Dec. lO, 1962 *I @la7/4 JNVENToR KENNETH W. EHLERS BY TTOR/VEX United States Patent C) PROCESS FR TREATIN METALLIC SURFACES WITH AN IONIC BEAM Kenneth W. Ehlers, Lafayette, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Fiied Dec. 1t), 1962, Ser. No. 243,630 3 Claims. (Cl. 117-93.3)

The present invention is a process for forcibly injecting gaseous substances into metal surfaces to produce coatings thereon which have specialized and highly useful properties. More particularly, the invention provides for the bombardment of metallic substances with energetic ion beams to produce surface films which may variously be alloys or compounds of the ions with the metal and which may exhibit an intense controllable color. The invention described herein was made in the course of, or under, contract W-7405-eng48 with the United States Atomic Energy Commission.

In the course of physics experiments which involve directed ion beams, such as are produced by the various forms of charged particle accelerator, it has frequently been observed that the ions mark their region of impingement on the target substance. Heretofore this marking has been ascribed to sputtering, or the break-up of molecules absorbed at the surface of the target material, and has not been investigated to any appreciable extent.

I have now found that the effects of free ions impinging on metals are not limited to sputtering, as heretofore understood, and that under getic ions may be directed into the surface of a metalic target substance to form a compound or alloy according to the particular ions and target material which are employed. The surface films so produced exhibit several unique properties. Most noticeably, the treated metal surface may be caused to exhibit an intense coloration which is uniform and reproducible and which may range throughout thefvisible spectrum according to the selected bombardment conditions. A further useful property of the process is that the depth of the layer formed on the metal may be controlled to a tolerance of less than one tenth of a micron. Still another desirable property of the process is that it may be employed to form films of substances not known to be obtainable by other means. It has been found possible, for example, to form interstitial compounds of such inert gases as 'helium and argon with metals such as aluminum.

The process may be performed by utilizing an ion source, linear accelerator or the like to produce a directed Broadly, the process is performed by disposing the metal to be treated in the ion beam in transverse relation thereto and in vacuum. For a given ion and metal, the color of the resulting coating is basically determined by the ion performed for the purpose of coloring metals, beam energy, beam current, and bombardment time are regulated to produce the desired effect. These conditions are also regulated to control other may be determined by control of the beam current and bombardment time.

Thus the invention provides a new technique for chemically or metallurgically combining substances and has other uses in addition to that 0f coloring metal surfaces. As will herein be discussed in more detail, the process has further applications which include storing gases in metals, the impregnation of metals with other elements to alter the mechanical or electrical properties of the base metal, analyzing the composition of ion beams, and the preparation of compounds and alloys not readily produced by other means.

it is therefore an object of this invention to provide a process for advantageously treating metallic materials by ion bombardment thereof.

It is another object of this invention to provide a process for forcibly injecting electrically charged gaseous substances into metals to form physical, chemical and metallurgical combinations thereof.

It is another object of the invention to provide a new and precisely controllable technique for forming surface coatings on metals for protective, decorative or other purposes.

It is another object of this process for selectively coloring metallic surfaces.

It is still another object of this invention to provide a process for producing metallic mixtures, alloys, compounds and the like which cannot be readily obtained by other means.

It is a further object of the invention to provide a more convenient and precise method for analyzing the composition of ion beams.

The invention, together with vantages thereof, will be better to the following description in companying drawing of which:

FIGURE l illustrates an arrangement of apparatus suitable for practicing the invention,

FIGURE 2 illustrates a metal the apparatus of FIGURE ment conditions,

FIGURE 3 illustrates a second metal treatment in the apparatus of FIGURE l set of conditions, and

FIGURE 4 shows a third plate, having bars of several metals mounted thereon,

further `objects and adnnderstood by reference conjunction. with the ac- 1 under a first set of bombardplate following under a second i films on metal surfaces which in thisy example were rectangular plates 11. The process requires a source 12 of the class which produces a directed output beam 13 of energetic ions. Suitable sources for this purpose are well maximum beam current available.

Under most conditions, an ion source 12 will simultaneously produce several different types of ion. This results in part from the fact that contaminants are present to the source and in part from the fact that most gases are capable of being ionized to several different charge states. In addition, charge exthe output beam 13. In order invention to provide a new plate after treatment in` eld 14. Field 14 may be produced by a conventional mass spectrometer magnet 16 and the action of the field on the beam 13 is similar to that occurring in a spectrometer. Thus the beam 13 is turned, in this instance 180, and separated into homogeneous sub-beams 13' each containing ions with a common mass, charge and energy. With the beam 13 separated in this manner, a flat bafe 17, having a slot 18, may be selectively positioned to pass only a selected one of the -sub-beams 13'.

The plate 1\1 which is to be treated is disposed transverse to the beam on the opp-osite side of bafiie 17 from field 14. If the beam 13 subtends less than the entire area of the plate 11, and it is desired to produce a coating throughout the whole area, the plate may be slowly scanned across the beam in the direction of arrow 19 for example.

The portions of the foregoing apparatus through which the beam 13 passes, from source 12 to plate 11, are enclosed by an airtight envelope 12 and a vacuum pump 22 is coupled thereto to evacuate the beam region. In some instances, it may be desirable to provide a cooling means at the back surface of plate \11 inasmuch as considerable heat is generated therein by prolonged ion bombardment.

The type of ion selected for bombarding the plate 11, as Well as the beam energy, beam current and bombardment time, are chosen according to factors which will hereinafter be discussed. For purposes of the present example however, a particular case will be discussed wherein the plate 11 was aluminum and the ion source 12 produced nitrogen ions using an extraction potential of 11.6 kilovolts. In this case, the total beam current was 13 milliamperes, the field 14 was 8 kilogauss, and the exposure time minutes. The plate 11 remained stationary throughout the bombardment and the bafille 17 was absent to allow each of the sub-beams 13 to mark separate zones of the plate.

Under the foregoing conditions, three distinct subbeams 13 were present consisting -respectively of N+, N24", and N31, Referring now to FIGURE 2, the appearance of the plate 11 following Vthe bombardment is shown. As indicated therein, the area 23 of the plate in the path of the N+ sub-beam was brown (olive toned). The N2+ yregion 24 was blue and the N3+ region 26 was yellow. The coloration is extremely bright and is permanent. In each instance, the colored film is a layer of aluminum nitride formed on the plate, the differences in color being due to optical phenomena which will hereinafter be discussed.

Nitrogen ions are ineffective in marking copper under the conditions of the foregoing example other than to produce a sputtered surface. However oxygen ion bombardment marks copper extremely well. Referring now to FIGURE 3, the appearance of a copper plate 11 is shown following treatment in the apparatus of FIGURE l where oxygen was supplied to the source and bombardment conditions included an extraction potential of l2 kv., a magnetic field of 7 kg., a beam current of 20 ma. and a bombardment time of l hour. Under these conditions, four distinct sub-beams are present, formed of and produce zones 27, 28, 29 and 31 on the plate which 'are colored brown, violet, blue and green respectively. The composition of the surface coatings in this case was CuO.

The results of a more complex bombardment are .shown in FIGURE 4. Pour parallel bars 32, 33, 34 and 36, formed of titanium, molybdenum, tantalum and beryllium-copper respectively, were mounted on an aluminum plate 1\1" which was positioned so that each sub-beam impinged on a portion of each bar. In this example, oxygen, nitrogen and helium ions were `simultaneously produced by the source under conditions which included an extraction potential of 10 kv., a magnetic field of 5 kg.,

a beam current of 25 ma. and an exposure time of 5 minutes. Sub-beams 37, 33, 39 and 41 of O+,N+, 02+ respectively Were present and produced different effects on the several metals. In the order given above, the ions produced areas on the titanium 32 which were brown, brown, uncolored, and violet. On the molybdenum bar 33 the O-iions produced an area colored Violet and all of the remaining sub-beams failed to mark the bar. Similarly, the O+ ions colored the tantalum bar 34 brown, the remaining ions failing to produce a color. In the order given above, the ions produced areas on the beryllium-copper bar 36 which were red, uncolored, blue and uncolored. In the same order, areas of the aluminum base plate 11" were marked brown, brown, uncolored and blue.

In the foregoing example, a predominance of browns and colors at the upper end of the spectrum, as well as the failure of several of the sub-beams to color several of the metals results from the relatively low energy of the ions `and the short exposure time under the specified bornbardment conditions, the basis for these effects, and the significance thereof being hereinafter discussed.

Further examples of ions, and the applicable bombardment conditions, which has been employed to produce coatings on metals are as follows:

Probable Composition of Product Energy Ion (in kev.)

Blue.

Olive-brown.

Green.

Brown.

Blue.

It should be understood that the foregoing list is not exhaustive of the surface coatings which can be formed by the process. In general, any ionized gas may be injected into any metal where the resulting layer material is stable and provided the other conditions herein discussed are met. The required stability of the surface film includes stability under the specialized conditions within the bombardment apparatus, i.e. high vacuum of the order of l0-6 mm. Hg and temperatures which may reach several hundred degrees centigrade. It is believed to be for this reason that nitrogen ions were unable to color copper as hereinbefore discussed. Nitrides of copper,

formed, would be extremely unstable in the described environment. This is also true of the nitrides and oxides of silver. Only sputtering marks are produced by the bombardment of an uncooled silver surface by oxygen or nitrogen ions.

The formation of films on the several metals by helium ions as indicated above is of particular interest inasmuch as it does not appear probable that true chemical combinations of gas with the metal would be produced. It is presently believed that the resultant surface layers are interstitial compounds or solid solutions.

There is a minimum exposure time for a colored mark to form, which is dependent on the current density of the ion beam as well as on its energy. The minimum time is that which is required to inject approximately one gas ion for each metallic atom to the desired layer depth. In aluminum for example a charge of approximately 0.1 millicoulomb per square centimeter must be delivered for each Angstrom unit of layer depth.

Considering now the basis for the formation of the colored films as presently understood, an ion in the energy range of about 5 to 50 kev. which strikes a metal surface positioned normal to its trajectory has sufficient energy to penetrate the lattice of atoms that comprise the metal surface and to lodge within. The impacting ion need only overcome barriers of the order of ev. to dislodge a target atom from its preferred location to an interstitial site, or to sputter away an atom located near the surface. After a period of bombardment the lattice consists of normal vacancies (Frenkel defects) plus many dislocations caused by the energetic ions. It is in the velocity region of these ions that the cross section for elastic nuclear collision becomes large and dominates the ionization cross section, and since the nuclear collision is effective in producing atomic displacement it is in this energy region that the maximum radiation damage can occur.

Thus the capability exists for the formation of interstitial compounds of the target metal and the impinging ion. Barring diffusion however, yall the possible effects and reactions between the metal and the incoming particle exist in a thin layer extending in from the surface of the target metal. The thickness of the layer is a function of the range energy of the particular combination of ion and target. If this thickness, with allowances for the index of refraction of the layer, is equivalent to a quarter of a wave length (or an odd multiple thereof) in the visible spectrum then reflection `at the layer-metal interface would make the layer appear colored as in Newtons rings. Assuming this to be the cause of the coloration produced by the present process, and assuming a unity index of refraction, the minimum film thickness which will produce a color other than brown or black is about 1000 A. This color is violet and if the film thickness is gradually increased by increasing the impacting ion energy, the color progressively changes toward the lower end of the visible spectrum to red indicating a film thickness of about 1750 A. If the film thickness is increased still further, the same progression of colors is produced each time the thickness moves through a range corresponding to an odd multiple of a quarter wave length of a color within the visible spectrum. The color will continu-ally become less pronounced, yas the film thickness is increased, due to the increasing opacity of the thicker film layer. Film thickness below about 1000 A. `and those intermediate between the color producing thicknesses described above, have a brown or black appearance due to destructive interference effects.

The foregoing hypothesis concerning the basis for the coloring of metals conforms with the results observed in the development of the invention and agrees fairly closely with the available theoretical predictions of the depth of penetration of ions into metals at the energy range of interest (see: K. O. Nielson, Electromagnetically Enriched Isotopes and Mass Spectrometry, Butterworths Scientific Publications, London 1956).

Regardless of what may be the exact cause of the observed effects, the proces-s provides a means for imparting a very attractive and precisely controllable coloring to metal surfaces, the resultant film also serving as 'a protective cladding in many instances. Moreover, the process may also be used for other purposes in which the production of color is a secondary purpose or is ineidental.

The process has proven to be extremely valuable, for example, in analyzing the composition and configuration of ion beams. Given a knowledge of the coloring produced by specific ion-metal combinations, a metal plate may be exposed to the beam whereupon the presence and approximate concentration of particular ions is detect- -able merely by inspection of the plate.

The process also provides a very convenient means for storing gases in metals in solid solution therein. For example, an uncooled copper plate was bombarded with 10 kev. He+ ions, and the marked area of the target removed for helium analysis. The total helium content of the copper sample was 1.78 micromoles which represented slightly less than 2% of the total amount of helium that had impacted the copper. During the analysis the evolution of helium from the sample was greatest in the temperature range of 700 to l000 C. and more than 99% of the total helium evolved was obtained before the copper sample melted. Assuming this amount of helium to have been contained in the known area of the copper to the depth determined by the theoretical considerations hereinbefore discussed, the volume of copper involved contained more than 2000 equal volumes (STP) of helium. The copper thus occluded 258 cm3 (STP) of helium per gram of metal. This is closely similar to the amount of hydrogen that can be occluded by titanium. Of particular interest is the fact that the helium is contained at a density greater than that of liquid. helium by a factor of approximately three. As the helium is most probably in solution, it is possible that sorne diffusion to a greater depth may have occurred during the short period of bombardment however this is very probably more than off-set by the amount of helium evolved from the target before the sample could be analyzed.

Inasmuch as the process provides a new technique for chemically and metallurgically combining an ionizable substance with a metal, in which the metal may remain in the solid state, other applications of the invention are possible. In the manufacture of solid state electronic circuit elements for example it is frequently necessary to impregnate the surface layer of a metallic crystal with an impurity in order to alter the electrical characteristics of the layer. Similarly, the manufacture of magnetic tape for information storage purposes requires the formation of an iron oxide film on the tape and certain of the electrical characteristics of the tape are improved in proportion to the thinness of the film. Other applications of the invention to processes requiring the 'formation of a very thin surface coating on metals will suggest themselves to those skilled in the art.

Thus while the invention has been disclosed with respect to certain illustrative examples, it will be apparent that many variations are possible within the spirit and scope of the invention and it is not intended to limit the invention except las defined by the following claims.

What is claimed is:

1. A method of analyzing the composition of an energetic charged particle beam of an inert gas and which has an energy in the range of 5 to 50 kev. comprising the steps of passing said beam through a transverse magnetic field to produce at least one division of said beam composed of ions having a common charge, mass and energy, and disposing a metallic plate in said beam in transverse relationship thereto for a period sufficient to inject a number of ions into a surface layer region of said plate which number of ions is about equal to the number of metallic atoms in said region whereby a coloration is imparted to said region of said plate that is indicative of the nature of said ions.

2. A process for forming an interstitial compound of an inert gas and a metal of the class capable of combining therewith comprising the steps of ionizing said inert gas, accelarating said inert gas into a directed ion beam, separating from said ion beam at least one beam of monoenergetic ions having an energy in the range of 5 to 50 kev. and a current in the range of tens of milliamperes, and disposing a plate formed of said metal in said monoenergetic ion beam in transverse relationship thereto whereby said -gas is forcibly injected into a surface layer of said plate to form said interstitial compound therein.

3. In a process for treating a metal surface to form a coating thereat, the steps comprising ionizing an inert gas, accelerating said ionized rgas into a directed beam, passing said beam of ionized gas through a transverse magnetic field to obtain at least one beam composed of monoenergetic ions having substantially a single charge and mass, disposing said metal surface in at least one of said beams of rnonoenergetic ions in transverse relationship thereto whereby said monoenergetic ion beam is injected into said metal surface and combines therewith to form said Coating, and regulating the energy level of said 5 monoenergetic beam Within a range of 5 to 50 ke'v., regulating the current level of said monoenergetic beam to tens of milliamperes and regulating the bombardment time of said monoenergetic beam to control properties of said coating.

References Cited UNITED ASTATES PATENTS 2,941,077 6/1960 Marker 250-49.5 3,192,892 7/1965 Hanson et al Z50-49.5 X 3,245,895 `4/1966 Baker et al 204-164 HOWARD S. WILLIAMS, Primary Examiner. JOHN H. MACK, Assistant Examiner. 

3. IN A PROCESS FOR TREATING A METAL SURFACE TO FORM A COATING THEREAT, TH STEPS COMPRISING IONIZING AN INERT GAS, ACCELERATING SAID IONIZED GAS INTO A DIRECTED BEAM, PASSING SAID BEAM OF IONIZED GAS THROUGH A TRANSVERSE MAGNETIC FIELD TO OBTAIN AT LEAST ONE BEAM COMPOSED OF MONOENERGETIC IONS HAVING SUBSTANTIALLY A SINGLE CHARGE AND MASS, DISPOSING SAID METAL SURFACE IN AT LEAST ONE OF SAID BEAMS OF MONOENERGETIC IONS IN TRANSVERSE RELATIONSHIP THERETO WHEREBY SAID MONOENERGETIC ION BEAM IS INJECTED INTO SAID METAL SURFACE AND COMBINES THEREWITH TO FORM SAID COATING, AND REGULATING THE ENERGY LEVEL OF SAID MONOENERGETIC BEAM WITHIN A RANGE OF 5 TO 50 KEV., REGULATING THE CURRENT LEVEL OF SAID MONOENERGETIC BEAM TO TENS OF MILLIAMPERES AND REGULATING THE BOMBARDMENT TIME OF SAID MONOENERGETIC BEAM TO CONTROL PROPERTIES OF SAID COATING. 